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SI. TITLE (include Security Classicaton) Orthorhombic-to-Tetragonal Transition in R +xBa2X Cu 307+6

'/ (R = Nd,Sm, and Eu)

12 PERSONAL AUTHOR(S) S. Li, E.A. Hayri, K.V. Ramanujachary, and Martha Greenblatt

13a. TYPE OF REPORT i3b TIME COVERED 14 DATE OF REPORT (Year, MonthOay) S PAGE COUNTTechnical I FROM 7/1/87 TO 7/15/8 July 15, 1988 5

16. SUPPLEMENTARY NOTATION

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The orthorhombic-to-tetragonal structural phase transition in the high-Tcasuperconducting oxiaes of the type Rj.xBazCu3O7+& [R (for rare earth) Nd,Sm, and Eul has been investigated using powder x-ray diffraction, dcresistivity, and thermogravimetric techniques. It was found that theorthorhombic-to-tetragonal transition occurs for samples whose nominalstoichiometric content of oxygen is greater than 7.0 (0<<0.3) as compared to

a clear convergence of multiple orthorhombic peaks to a well-defined single (Ii

tetragonal peak was observed in the x-ray diffraction pattern. The presenceof orthorhombic distortion in this system appears to be essential for

achieving 90-K superconductivity. e,. ., , ,,0, ." ,", ,

20 D:ISTR.8QTON AVAILABILTY OF ABSTRACT 21 ABSTRACT SEC..RTY C ..ASSFCATiON-' NCLASSiFIED/UNLMITED 0 SAME AS ;PT C3 D- C _SEQS

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ORTHORHOMBIC-TO -TETRAGONAL TRANS ITIONIN Rel1 +.Ba2 -. CU3O,+6 (Re-Nd, Sin, and Eu)

S. Li, E. A. Hayri, K.V. Ramanujachary, and Martha Greenblatt*Department of Chemistry

Rutgers, The State UniversityNew Brunswick, NJ 08903, USA

Accession For

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*Author to whom communications should be addressed

The orthorhombic-to-tetragonal structural phase transition in the high Tc

superconducting oxides of the type Rel+1Ba2 -Cu 307+6 (Re-Nd, Sm, and Eu) has

been investigated using powder X-ray diffraction, D.C. resistivity, and

thermogravimetric techniques. It was found that the orthorhombic-to-tetragonal

transition occurs for samples whose nominal stoichiometric content of oxygen

is greater than 7.0 (0 < 6 < 0.3) as compared to less than 7.0 in YBa2 Cu3O _6.

With increasing Re/Ba ratio in Rej+,Ba2-_Cu30746 a clear convergence of multiple

orthorhombic peaks to a well defined single tetragonal peak was observed in

the X-ray diffraction pattern. The presence of orthorhombic distortion in this

system appears to be essential for achieving 90 K superconductivity.

2

INTRODUCTION

It is well established that the high T, superconducting oxides LnBa2CU 3O7_6

(referred to as 123 compounds, where Ln-Y and all the rare earth elements

except Ce, Pr, and Th) undergo an orthorhombic-to-tetragonal transition as a

result of variation in the oxygen content and oxygen distribution. T, is

dramatically effected by oxygen content, oxygen distribution, and crystal

symmetry.1.2 For 6-0 the crystal symmetry is orthorhombic and T,-92 K. When

samples are heat treated at elevated temperatures and/or in reducing

atmospheres the oxygen content and Tc decrease; at 6 > 0.5 the samples may be

tetragonal and semiconducting. The depletion of the oxygen content of 123

compounds also leads to the reduction of both the formal oxidation state and

coordination number of the Cu(l) atoms in the Cu-0 one-dimensional chains

along the b direction. In the fully oxygenated orthorhombic form of 123, the

average formal valence of copper is 2.33 and all of the 0(4) (0, 1/2, 0)

positions are occupied, while all of the 0(5) (1/2, 0, 0) positions are

empty.3 In tetragonal YBa2Cu306, Cu(l) appears to have a formal charge of +1

and 2-fold coordination to oxygen along the C axis of the unit cell. 4 The

tetragonal form of 123 is semiconducting, while the ordered orthorhombic

YBa2Cu306.3 is superconducting;5.6. in the former, the 0(4) and 0(5) positions

are randomly occupied, in the latter, there are an insufficient number of 0(4)

atoms along the b axis, and thus long range Cu-0 chain formation is disrupted.

YBaZCu 307_6 with 0.3 < 6 < 0.5 prepared at low temperature by oxygen getter

methods is a -60 K bulk superconductor.2 ,5 Thus both oxygen content and the

microscopic oxygen configuration has a large effect on Tc, and near full (6 <

0.2) occupation of the 0(4) positions is required for 90 K superconductivity.

The structural and transport properties of orthorhombic/tetragonal YBa2Cu307_6

phases are well established.

3

~ '. -

In addition to thermal treatments, the oxygen content and the crystal

symmetry of the the 123 compounds may be changed by chemical subtitution. For

example, in La1jBa2 -1 Cu307+6 , La34 substitution for Ba2 leads to an increase in

the oxygen content, a change in the distribution of oxygen ions in the

lattice, and concomitant changes in crystal symmetry, electronic properties

and T,.6 .7 For 0 < x < 0.3 the samples are orthorhombic and superconducting,

while for x > 0.3, the samples are tetragonal and semiconducting. 6 increases

with increasing x, however, the formal oxidation state of copper is 2.33,

nearly independent of the oxygen content.6 This suggests that the copper

oxidation state alone is not sufficient to produce superconductivity. More

recent reports indicated that even small rare earth ions including Nd, Sm, Eu,

and Y can substitute for the large Ba cations in the 123 structure leading to

higher oxygen content than seven.8-1 0 However, in these reports, the

relationship between the oxygen content, orthorhombic-to-tetragonal phase

transformation, and superconducting behavior was not established in detail.

Substitutions for Cu by all of the 3d transition metal cations or by Al3 + or

Ga3+ have also been carried out; some of these substitutions also lead to

orthorhombic-to-tetragonal phase transitions. Nevetheless, it is not clear at

the present, what the effect of 3d transition metal or that of the Al3 + or Ga3

ion substitutions are on the oxygen content or oxygen ordering of the 123

compounds. 11-13

We have undertaken a systematic investigation of Re(Ba..Rex)Cu30 7+6 with

Re-Nd, Sm and Eu in order to: 1. examine the range of x for solid solution

formation and its relationship to oxygen content, copper valence and high TV

superconductivity; 2. to find unambiguous evidence of orthorhombic-to-

tetragonal transition in these substituted phases and to establish the

relationship between oxygen content and symmetry transformation. In this

communication we show an upper limit of oxygen content for the existence of

4

I ........

the high T, superconducting phase and unambiguous evidence of orthorhombic-

tetragonal transition in Re(Ba2_xRe.)Cu 307+5 with Re-Nd, Sm and Eu; the

transition is sharp and occurs at x-0.2 . The oxygen content increases, while

T, decreases with increasing x.

EXPERIMENTAL

Rare earth oxides used in this investigation were fired at 950°C in air

to eliminate hydrates, carbonates, and other impurity adsorbates.

Stoichiometric amounts of reagent grade, or better purity Nd203 or Sm203 , or

Eu203, BaCO3 and CuO were weighed according to the chemical equation:

(l+x)/2Re2 O3 + (2-x)BaCO3 + 3CuO - Re 1 +xBa2.. Cu3OY

The mixtures were ground in an agate mortar and calcined in air at 9500 C with

repeated grindings and refirings (usually two or three), until no changes in

the powder X-ray diffraction could be detected. The powder samples were

pressed into pellets and then sintered at 950°C for 24 hrs. In order to

maximize the oxygen content, pellet samples were annealed at 450-5000C in

flowing oxygen atmosphere for 24 hr, followed by slow cooling to room

temperature. X-ray powder diffraction data were recorded by a SCINTAG PAD IV

diffractometer using Si as an internal standard. Oxygen contents were

determined by H2 reduction of the powder specimens in a DuPont 951

thermogravimetric analyser (TGA). Electrical resistivity was measured in the

temperature range 4-300 K on rectangularly shaped bar samples with indium

solder contacts in a four probe configuration. All measurements reported in

this investigation are reproducible.

RESULTc AND DISCUSSION

X-ray powder diffraction data indicate that the solubility limit of

Re(Ba2.,Rex)Cu3)7+6 with Re-Nd, Sm, and Eu is 0 : x 50.5. The prediction of

Zhang et al, 9 of a larger upper limit of solubility in Re(Ba2_xRex)Cu 307+6 with

increasing size of the rare earth ion was not observed. For compositions

, . I . " I-- - - - - -a

corresponding to x-0.5 the powder X-ray diffraction pattern of Re(Ba2-

xRe,)Cu3 76 analogs show close resemblance to that of La 3 Ba3Cu60 1 +6 (336). Fig.

1 compares the diffraction patterns of Nd(Ba 2 .xNd)Cu3 07 +6 of x-0.0 and x-0.5.

It is evident that the 336 analogs of Sm, Nd, and Eu are isostructural with

their parent 123 structures in agreement with recent neutron and X-ray

diffraction studies of Ndl+xBa 2 _-CuOy. l' ,l 5 At x > 0.5 decomposition of the

perovskite-type phase occurs, and an impurity phase of K2NiF4 -type shows up in

the powder diffraction pattern of Sm and Eu at x-0.6; an unidentified phase is

seen in the Nd system at x-0.6.

Table I summarizes the unit cell parameters, crystal symmetry, total

oxygen content, and T. for the series Re(Ba2._Rex)Cu 307+6, (Re-Nd, Sm, and Eu).

Cell parameters were determined by fitting the observed X-Ray data by least

square refinement techniques. Orthorhombic Nd(Ba2._Ndx)Cu307+6 , Sm(Ba2_

XSm)Cu 307+6, and Eu(Ba 2 _xEu)Cu 307+6 show a decrease in the b and c cell

parameters, and an increase in the a parameter with increasing Re/Ba ratio.

a and b converge for x-0.2 (Fig. 2), then decrease monotonically. Thus the

orthorhombic-to-tetragonal phase transition is clearly resolved in all three

systems. Peak profiles of the (0 0 6), (0 2 0 ), and (2 0 0) reflections as a

function of x in Nd(Ba2.xNd.)Cu3 07 +6 are presented in Fig. 3. For x-0, the

characteristic orthorhombic splitting of the peak is seen. With increasing x,

the triplet peak gradually transforms first to a doublet and eventually to a

single peak at x-0.5. Similar behavior is seen in the Sm and Eu analogs,

except that the tetragonal phase seems to be stabilized for smaller values of

x (-0.1-0.2).

Fig. 4 shows the variation of the total oxygen content in Nd(Ba2 -

,Nd)Cu3 0+ 6 as a function of x as determined by TGA. A nearly monotonic

increase in 6 with increasing x is observed. In all three systems we see a

clear transition from orthorhombic-to-tetragonal symmetry at 6-0.10±0.01. This

6

JM N.

indicates that a minimum occupancy of the 0(5) site is required to increase

the symmetry. At low values of 6 all of the 0(4) sites, and few of the 0(5)

sites are occupied so that orthorhombic symmetry and long range order of the

one-dimensional Cu-0 chains in the b direction remains, mediating

superconductivity. However, at higher values of 6 with more of the 0(5) sites

being occupied, the structural transformation to tetragonal symmetry occurs;

the chains are partially replaced by Cu-0 octahedral layers in the basal plane

(ab) and superconductivity is destroyed. A recent report suggests that by

annealing the 336 samples under high oxygen pressure, the 0(5) occupancy might

be increased up to 6-0.6 with superconductivity observed in the sample at -30

K.16 However, this result needs to be confirmed by others.

The temperature dependence of resistivity is shown in Fig. 5 for Re(Ba 2_

.Re.)Cu307+6 with Re-Nd, Sm for the range 0 - x ! 0.5. For x-0, (6-0) metallic

behavior between 300-90 K and a metal-to-superconductor transition at 90 K are

observed. The room temperature resistivity values scale linearly with x. A

local minima is evident before the onset of superconductivity for compositions

with 0.2 < x < 0.4 for Nd and with 0.1 < x < 0.3 for the Sm compounds. Fig. 6

indicates the variation of TC with x for the Nd and Sm series of solid

solutions. T, decreases with increasing x for both in a similar way. When x

0.4 for the Nd and x ? 0.3 for the Sm series only semiconducting behavior is

seen down to 4 K. The Eu compound is still superconducting at x-0.4 at low

temperature (Table I). These results indicate that the tetragonal phase has a

deleterious effect on the superconducting properties in these systems

providing further evidence that square planar coordination of Cu(l) in the bc

plane is essential for superconductivity. The metal-to-semiconductor

transition and the broadening of the superconducting transition seen in some

of the substituted samples (Fig. 5) are attributed to inhomogenieties of the

samples. Part of the imhomogeneities might be due to differences in the

relative occupancy of the 0(4) and 0(5) sites in different regions of the

7

pellet specimen. However, it might be partly due the magnetic rare earth ions

(Nd, Sm, Eu) on the Ba2+ site effecting superconductivity.

In summary, we have found solid solution formation in Rel+,Ba 2 _,Cu 3 O7 +6

(Re-Nd, Sm, and Eu) for 0 : x :5 0.5. With increasing x the oxygen content

increases and the formal oxidation state of Cu remains -2.33. A clear

orthorhombic-to-tetragonal phase transition at x-0.2 is observed. Tc decreases

with increasing oxygen content.

We wish to acknowledge helpful discussions with Dr. S. Fine. This work was

supported by the Office of Naval Research and by the National Science

Foundation Solid State Chemistry Grants DMR-84-04003 and DMR-87-14072.

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REFERENCES

1. J. D. Jorgensen, B. W. Veal. W. K. Kwok, G. W. Crabtree, A.

Umezawa, L. J. Nowicki, and A. P. Palikas, Phys. Rev. B 36, 5731

(1987).

2. R. J. Cava, B. Batlogg, C. M. Chen, E. A. Rietman, S. M.

Zahurak, and D. Werder, Nature 329, 423 (1987).

3. P. K. Gallagher, M. M. O'Bryan, S. A. Sunshine, and D. W.

Murphy, Mat. Res. Bull. 22, 995 (1987).

4. A. Santoro, S. Miraglia, F. Beech, S. A. Sunshine, D. W. Murphy,

L. F. Schneemeyer, and J. V. Waszczak, Mat. Res. Bull. 22, 1007

(1987).

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and D. Werder, Phys. Rev. B 36 5719 (1987).

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Jorgensen, M. A. Beno, and I. K. Schuller, Nature 329 227

(1987).

7. S. A Sunshine, L. F. Schneemeyer, J. V. Waszczak, D. W. Murphy,

S. Miraglia, A. Santoro, and F. Beech, J. Cryst. Growth 85, 632

(1987).

8. Z. Iqbal, F. Reidinger, A. Bose, N. Cipollini, T. J. Talor, H.

Eckhardt, B. L. Ramakrishna, and E. W. Ong, Nature 331 326

(1988).

9. K. Zhang, B. Dabrowski, C. U. Segre, D. G. Hinks, I. K Schuller,

J. D. Jorgensen, and M. Slaski, J. Phys. C: Solid State Phys.

20, L935 (1987).

10. T. Iwata, M. Hikita, Y. Tajima, and S. Tsurumi, Jpn. J. Appl.

Phys. Lett. 26, L2049 (1987).

9

q .,,,~~ a,,-% , - P ! a a i'\a V V araf' %'% ' %

11. Y. Maeno, M. Kato, Y. Aoki, and T. Fujita, Jpn. J. Appl. Phys.

Lett. 26, %1982 (1987).

12. T. Siegrist, L. F. Schneemeyer, J. V. Waszczak, N. P. Singh, R.

L. Opila, B. Battlog, L. W. Rupp, and D. W. Murphy, Phys. Rev. B

36, 8365 (1987).

13. 1. Sankawa, M. Sato, and T. Konaka, Jpn. J. Appi. Phys. Lett.

26, L1616 (1987).

14. F. Izumi, S. Takekawa, Y. Matsui, N. lyi, H. Asano, T. Ishi, and

N. Wa,-anabe, Jpn. J. Appi. Phys. 26, L1616 (1987).

15. K. Takita, H. Katoh, H. Akinaga, M. Nishino, T. Ishigaki, and H.

Asano, Jpn. J. Appi. Phys. Lett. 27, L57 (1988).

16. S. Tsurumi, T. Iwata, Y. Tajima, and M. Hikita, Jpn. J. Appi.

Phys. Lett. 27, L80 (1988).

10

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TABLE I. Physical Parameters of Rel, 1 Ba 2..1CU3 07+6 , Re-Nd, Sm and Eu

N-d +.-B-A2 - Cu0+ 6

Comp. Cryst. Cell Parameters (A)x Sym. a b c Tonat (K) TCZ~rO (K) 6

0.0 0 3.871(2) 3.914(1) 11.756(2) 88 77 0.040.1 0 3.871(1) 3.914(3) 11.7321(1) 81 50 0.050.2 T 3.890(1) 3.892(2) 11.696(2) 54 33 0.100.3 T 3.890(2) -11.661(1) 50 14 0.140.4 T 3.874(3) -11.659(4) -- 0.190.5 T 3.876(3) -11.649(1) -- 0.30

LMni. .B22-ig-U327+6

0.0 0 3.858(0) 3.910(0) 11.741(0) 92 82 0.010.1 0 3.860(2) 3.906(2) 11.729(3) 87 70 0.050.2 T 3.881(0) - 11.654(2) 47 29 0.090.3 T 3.879(2) - 11.630(2) - - 0.160.4 T 3.871(1) - 11.599(2) -- 0.190.5 T 3.861(2) - 11.603(3) -- 0.22

Euj+xBa7,,,CU3 Q7 +6

0.0 0 3.844(1) 3.904(4) 11.709(4) 92 88 0.010.1 0 3.854(3) 3.887(8) 11-679(6) 92 70 0.070.2 T 3.873(0) -11.631(2) 56 28 0.180.3 T 3.867(1) -11.624(2) 53 26 0.150.4 T 3.873(3) - 11.619(2) 43 13 0.160.5 T 3.859(1) 11.579(3) - - 0.32

111

FIGURE CAPTIONS

FIGURE 1. Comparison of powder X-ray diffraction patterns of two members of

the solid solution series Ndl+Ba 2 _.Cu 307+6 (a) x-O.O; (b) x-O.5

(Nd336).

FIGURE 2. Variation of the cell parameters a, b, and c as a function of x in

Ndl+xBa 2-xCu 3O7 +6 .

FIGURE 3. X-Ray diffraction peak profiles of the (2 0 0), (0 0 6), and (0 2

0) reflections of Ndl+,Ba 2 _,Cu 3O7 +6 as a function of x.

FIGURE 4. The oxygen content, 6 as a function of x in Ndl+xBa 2 -CU 3 0 7 +6.

FIGURE 5. Temperature dependence of the resistivity as a fuction of

temperature in Rel+1Ba 2-xOCU3 7 +6 . (a) Re-Nd; (b) Re-Sm.

FIGURE 6. TC as functionof x in the solid solution series Ndl+xBa2 ,Cu3 07+6

(a) and Sm1+xBa 2 xCU 3O7 +6 (b); * :Tconse; o :T z ero

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